Volkswagen admitted to possible anti-competitive behaviour in a letter it sent to cartel authorities on July 4, Der Spiegel said.

The carmakers discussed their choice of suppliers and the price of components. Since 2006, the carmakers have also discussed the cost of AdBlue, an exhaust emissions treatment system for diesel engines, it said.

They discussed details such as the sizing of tanks for diesel emissions treatment fluid and they agreed to use smaller rather than larger ones, Der Spiegel said.

Daimler which owns the Mercedes-Benz brand, declined to comment.

A spokesman for Volkswagen, which owns the Porsche and Audi brands, also declined to comment. BMW was not available for immediate comment. (Reporting by Edward Taylor; editing by Jason Neely)